One general problem of driven machines, such as delivery systems, e.g., pumps, compressors and fans, or such as mills, crushers, vehicles, etc., is efficient variable-speed operation, or starting under high load, since, e.g., electrical machines, but also internal combustion engines, in most cases have a lower starting torque than their rated torque. In addition, electrical machines are used as the example for prime movers, but the principle applies to all possible types of prime movers, such as, e.g., internal combustion engines.
The most frequently used electrical inputs are currently three-phase machines, such as, e.g., asynchronous motors and synchronous motors. In spite of high electrical power consumption, three-phase machines at rest are not able to deliver this power fully mechanically; this is reflected in high losses and a low starting torque. At the same time, the current consumption of a three-phase machine when starting from speed zero typically corresponds to roughly 7 times the rated current; this causes a correspondingly high electrical load for the grid during starting.
Therefore, a three-phase machine must be designed to be correspondingly large so that it can deliver an input torque that corresponds to the rated torque from rest and is for this reason often oversized. Also for this reason, electrical machines are therefore often designed in combination with a frequency converter as a variable-speed input instead of being connected directly to a grid. Thus, starting with high torque from speed zero can be implemented without loading the grid; the approach is, however, expensive and associated with major losses of efficiency. One alternative that is more cost-favorable and also better with respect to efficiency compared to this is the use of differential systems—for example according to AT 507394 A. The fundamental limitation here, however, is that depending on the transmission ratio of the differential stage, only a relatively small speed range or, in the so-called differential mode, essentially no low speeds can be achieved on the drive shaft of a driven machine.
There are various possibilities for doing this. According to German Utility Model DE 20 2012 101 708 U, for example, the transmission ratio of the differential gear system can be fixed at 1. On this basis, the complete drive train can be driven with the differential drive or the prime mover can be brought to synchronous speed, and the latter can subsequently synchronize with the grid.
The disadvantage of this approach is that the differential drive or its frequency converter is significantly smaller-sized than the prime mover and therefore can also only deliver a correspondingly small torque.